The present invention relates to detectors and, more particularly, to detectors capable of detecting the presence of a gas by measuring a reduced transmission of a radiant energy caused by its absorption by the gas in a transmission path.
Gas detectors are known in which an emitter projects a beam of radiant energy through a column of a gas to be examined. A detector responds to radiant energy emerging from the column of gas to produce an electrical signal. The column of gas absorbs the radiant energy at wavelengths characteristic of the type of gas and with an absorption magnitude proportional to the amount of the gas in the column of gas.
Some gas detectors are employed to detect the presence of a particular gas while ignoring absorption from all other gasses. Such discrimination between desired and undesired gasses is attained by employing a selective filter in the beam of radiant energy effective to pass that portion of the spectrum in which the target gas is known to absorb radiant energy and to block substantially all other portions of the spectrum. Provided that the absorption spectrum of the target gas is reasonably well isolated in wavelength from the spectra of other gasses which might exist in the atmosphere, such a single-gas detector is quite effective.
The present invention seeks to provide apparatus for detecting the presence of a plurality of related gasses such as, for example, a plurality of fluorocarbon gasses. Such fluorocarbon gasses have absorption characteristics exhibiting overlapping absorption spectra. Thus, detection is complicated by interference among the overlapping spectra. For detection of a single pair of gasses, one might employ a filter positioned at a wavelength coinciding with the crossover between the two absorption spectra. The detector output in such a system is a reasonable approximation of the gas concentration, without concern for which one of the pair of gasses produces the absorption.
When the number of gasses to be detected rises to more than two, the spectra are generally so disordered that it has generally been considered impossible to find a single spectral region in which absorption from all of the gasses of interest are about equal.
In one application, any one or more of as many as four or five fluorocarbon gasses may escape to the environment. In a shipboard environment, for example, fluorocarbons 11, 12, 113 and 114 may be used. Fluorocarbons, being heavier than air, may accumulate in lower portions of the vessel. Since they are colorless and odorless, their presence may not be detected. Personnel working in an environment containing a significant concentration of any of the fluorocarbons may suffer oxygen deprivation and, in an extreme case, may become asphyxiated.
From the standpoint of personnel safety, all of the fluorocarbons of interest have about the same effect. That is, a concentration of fluorocarbon 11, for example, has about the same effect as the same concentration of any of the other fluorocarbons of interest. Furthermore, the presence of two or more fluorocarbons are additive in their effect. Two of the above fluorocarbons, both having the same concentration, produce about the same physiological effect as twice the concentration of either.
It would thus be desirable to have a fluorocarbon detector responsive to the fluorocarbons of interest, either alone or in any additive combination. Such a detector should have an alarm device responsive to the total concentration of all of the fluorocarbons of interest, and be substantially non-responsive to other gasses expected to co-exist in the environment.